-
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Introduction 1O. Ratib and M. Schwaiger
-t>
Contents PET/MRI in Clinical Practice
PETfMRl in Clinieal Practiee .. Following the path of elinical
applications of hybrid PET/CT it would see m logical that PETlMR
can provide an inno-vative and attractive alterrative taking full
advantage of thesuperiority of MR over CT in differentiating soft
tissue char-acteristics with a reduction in radiation exposure by
replac-ing CT with MR imaging. The challenges of PET/MR arestili
numerous both on the tcchnical side as on the practicaland elinical
side.
From PET/CT to PETfMR ...
Potentials and Challenges of PETfMR ..
The Challenge of Hyhrid Irnaging Protocols ...
Domains of Clinical Applicatios of PETfMR .
Future of PETfMR in Oncology .
4
4
4
Furthcr Reading ...
From PET/CTto PET/MR
The development of PET/MR started even before the
firstprototypes of PET/CT were developed. As deseribed indetails in
the next chapter, the technical challenges were con-siderable to
overcome the interference and cross-talk effectsbetween magnetic
field and the photomultipliers of PETdetectors. Several
alternatives have emerged and lead to thefirst hybrid devices to
appear on the market for elinical appli-catiors. Whether it is
through a combination of separatecoplanar systems or by integrating
solid-state PET detectorsinside an MR! they provide the means to
explore the poten-tial elinical applications of whole-body PET/MR
in elinical
practice.Given the broad ran ge of applications of PET
imaging,
it is oneology that remains today the elinical domain wherePET
is mostly used. PET imaging was shown to be supe-rior to other
imaging techniques in staging and follow-upof numerous specific
tumors. The advent of PET/CT hasreinforced the elinical utilization
of PET by a!lowing com-bined PET and CT studies to be acquired
quasi-simultane-ously with perfect alignment of anatomical and
metabolicimaging data. While most elinical studies showed
rela-tively modest improvement in diagnostic accuracy throughthe
sensitivity and specificity of hybrid PET/CT over PET
O. Ratib(tSl)Department of Medical Imaging and Information
Sciences.Divisior of Nuclear Medicine and Molecular lmaging,Geneva
University Hospitals, Geneva. Switzerlande-mail:
[email protected]
M. SchwaigerKlinikum rechts der IsarNuklearmedizinische Klinik
u. Poliklinik, Techrische UniversitatMrchen, Munich, Germany
::'!l!r~~O. Ratib et aL.(eds.). Arlas of PET/MR lnaging in
Oncology,DO! 10.1007/978-3-642-31292-2_1, Springer-Verlag Berlin
Heidelberg 2013
-
4O. Ratib and M. 5chwaiger
and CT performed separately, several studies demonstratedhowever
a significant improvement in diagrostic confidencewhen both studies
were acquired together and images wereinterpreted with fusion of
both modalities. Uncertainties ofdiagnostic findings that could
occur when PET image s areinterpreted without accurate anatomical
localizatior canbe avoided when CT images are co-registered with
PETimages and provide the necessary anatemical
references.Conversely, diagnostic criteria of CT based on
structural andmorphological observations can be of ten misleading
andsubject to difficult interpretation and can be
significantlyimproved when additional metabolic information of PET
isprovided with combined PET/CT images. While MRI hasreplaced CT in
many elinical applications by providing abil-ity for higher tissue
characterization and funetional irnag-ing, the added value of
PETlMR over PET/CT stili remainsto be dernonstrated in elinical
practice. The most immediateapplication might be in patients that
require both PET/CTand MRI in their elinical workup and could
benefit froma single study combining PET and MR when CT is of
nosignificant additicnal value.
Patentials and Challenges of PET/MR
Technical designs of hybrid PETlMR devices differ corsid-erably
between differert vendors as shown in the next chap-ter of this
book. While integrated system my represent themost logical solution
they retnain technically more challeng-ing and copianar systems
combining two scanners adjacentto each other provide an alternative
that resembles in itsdesign current PET/CT systems where the
patient is rnovedfrom one to the other modality sequentially. In
oneologyapplications both sequential and simultaneous aeqisition
ofirnages may provide similar results they differ only on
theirnaging protoeols used,
The rernaining challenge of all hybrid PETlMR systemsis the
calculation of tissue auenuation correetion maps sirni-lar to those
calculated from whole body CT seans in hybridPET/CT devices.
Several techniques of attenuation correc-tion were explored, but
the emphasis has been mainyon MRimage segrnentaion for dervation of
an attenuation map,Besides image segmentalion, other technical
challenges foreffective attenuation correetion in a whole-body
PET/MR,including compensation for MR image truncation and
cor-reetion for RF eoils and aeeessories also need to be
irnple-mented. Accurate caleuIation of tissue attenuation
airnsmainly toward providing quantitative measurement of PETracer
uptake in tissue. Semi-quantitative ealeulation of stan-dard tracer
upake (SUV) is the most comman techriqueused today and provides an
attractive simple method to
estimate the amount of tracer uptake in different tissues.While
it can allow for differentiating between benign physi-ological and
potentially pathologieal tissue tracers uptake, ithas only marginal
added value in routine elinical practicewhen PET findings are
combineo with observations fromother imagirg modalities such as CT
and MR and confrontedwith multiple other criteria in elinical
decision making. Itrernains however necessary to achieve the best
and mostaccurate correction of ussue auenuution to use PET
imagingtechnology at its full potential and berefit from the
addedvalue of quantitative imaging over visual interpretation
of
PET findings.
The Challenge of Hybrid Imaging Protocols
Depending on the scanner design, imaging protocols can dif-fer
significantly depending on the ability to perforn sequer-tial or
simultaneous acquisitiors of both modalities. ltremains however
that one of the most challenging aspect ofhybrid PETlMR is the
complexity and heterogeneity of MRprotocols that are being used in
elinical practice. The wealthof different types of imaging
parameters that MR! providesand the diversity and lack of
standardizetion of differentimaging protoeols have lead to
significant differences in pro-tocols used in elinical practice.
The general trend is that MRimaging protocols have considerably
been extended toinclude several sequences for better tissue
characterizationand extraction of functional and physiological
pararneters ofdifferent issues and organs. The combination for such
com-plex protocols together with additicnal whole-body
imagingsequences of PET and MRI can lead to significantly
longerexaminatior time. Therefore, additional efforts and
researchis needed to opimize hybrid imaging protocols to
benefitfrom the best potential capabilities of euch modality
whilemaintaining reasonable imaging time that is cornpatible
wihroutine elinical pracice.
Domains of Clinical Applications of PET/MR
The prirnary elinical applications of PET/MR that we electedto
cover in this book are in oncology. But there are otheremerging
applications in cardiovascular, n inflammatoryand infectious
disease as well as in neurodegenerative dis-eases. MRI has gained a
wide adeption in cardiology for thedetection of ischemic disease,
myocardial viability ad car-diac function. The added value and
complementarity of PETin providing a better sensitivity and
quantitative analysts ofmyocardial perfusion and myocardial
viability can becornegood elinical justifications for combineo
PETlMR studies.
1 Introduction
Further ReadingBrain imaging for acute as well as chronic
disease of ten relyon combination of multiple imaging modalities
such as PETand MR. In brain imaging however, the rigidity of the
headand the easily identifiable skull structures, allow
software-based registration technique to provide adequate fusion
ofdifferent rnaging modalities. A wider availability of
hybridPETlMR can however facilitate the use of hybrid imag-ing for
brain studies with more eonvenience to the patientthat does not
have to undergo separate studies on different
scanners.Arother factor that favors MRI over CT for hybrid
imag-
ing is the reduction -in radiation exposure. Although forelderly
patient and patients under palliative treatment forcancer,
radiotion exposure may be of minor impact, redueedradiation
exposure of PETIMRI compared with PET/CT maybe relevart in
non-oneological patients and in youngerpatients with poterially
curable disease.
Future of PET/MR in Oncology
This book highlights the potertial applications of whole-body
hybrid PETlMR in oncology. While it is stili a collec-tion of
convincing aneedotal cases, it only reflects earlyobservations of
two acadernic centers that were first in adopt-ing this new
technique n elinical praetice. The diversity ofcases and broad
scope of elinical domains covered in the dif-ferent chapters of the
book underline the potential applica-tions in oneology but alsa in
other elinical dornains, The useof different radiolabeled tracers
such as 18F-fluorocholineand 'F-ftuorotyrosine show the potential
of PET beyondthe conventional 18F_FDG tracer n elinical
applieations ofhybrid PET/MR. However this new imaging rnodality
hasonly been recently introduced for elinical use and will facethe
same challenges and skepticism that PET/CT techniqueeneountered
when it was first introduced. The lack of tangi-ble added value of
corrbired PET/MR over the two exami-nations acquired separately is
the first issue that needs to beaddressed both from a elinical
perspective and from a med-ico-ecoromic point of view. it is
however foreseeable thatinerensing demand for objective criteria in
determination ofadequacy and efficacy of new treatments, in
particnlar inoneology. will drive the development of new tracers
andinnovative hybrid imaging protocols that take full advantageof
complementarity of PET and MR modalities.
Antoch G, Bockisch A (2009) Combined PETIMRl: a new dirnensionin
whole-body oneology imaging? Eur J Nucl Med Mol Imaging
36Suppll:SI13-St20
Aruoch G. Saoudi N. Kuehl H et al (2004) Accuracy of
whole-bodydual-modality ftuorine-18-2-ftuoro-2-deoxy-O-glucose
positronernissiou tomography and computed tomography
(FDG-PET/CT)
~ for tumor staging in solid tumors: comparison with cr and PET.
J Clin Oncol 22:4357-4368
Bar-Shalom R. Yefrernov N. Guralnik L et al (2003)
Clinicalperfonnanee of PET/cr in evaluntion of cancer: additicnal
valuefor diagnostic imaging and parient management. J Nud Med
44:1200-1209
Beyer T. Pichler B (2009) A decade of conbined imaging: from a
PETattached to a CT to a PET inside an MR. Eur 1 Nucl Med
Mollmaging 36 Suppl I:S I-S2
Beyer T. Weigert M. Quick HH et al (2008) Mk-based aueruauon
cor-rectior for torso-PETI1vIR imaging: pitfalls in mapping MR to
CTdata. Eur J Nucl Med Mol Imaging 35: 1142-1146
Che n W. Jian W, Li HT e al (2010) Whole-body
diffusion-weighedimaging vs. FDG4PET for the detection of
non-small-cell lung can-cer. How do they measure up? Magn Reson
Imaging 28:613-620
Collins CD (2007) PET/cr n oncology: for which tumours is it
thereferenee standard?Cancer Imaging 7 Spec No A:S77-S87
Czemin J. Allen-Auerbach M. Schelbert HR (2007) Imprcvements
incancer staging with PET/CT: literaure-based evidence as
ofSeptember 2006. J Nucl Med 48 Suppl I :78S-88S
De1so G. Ziegler S (2009) PET/MRI system design. Eur J Nucl
MedMol Imaging 36 Suppl t:S86-S92
Hcusncr TA, Kuemmel S, Umutlu L et al (2008) Breas canccr
stagingin a single session: whole-body PET/cr mammography. J
NuclMed 49: 1215-1222
Heusner TA. Kuemmel S. Kocninger A el al (2010) Diagnostic value
ofdiffusion-weighted rragnetic resonance imuging (DWl) comparedto
FDG PET/cr for whole-body breast cancer staging. Eur J NuclMed Mol
Imaging 37(6): 1017-1086
Hofmann M, Pichler B. Seholkopf S. Beyer T (2009) Towards
quantita-tive PET/MRI: a review of Mp-based attenuation correction
tech-niques. Eur J Nucl Med Mol Imaging 36 Suppl I:S93-S104
Hu Z. Ojha N. Renisch S et al (2009) Mfc-based attenuarion
correc-tion for a whole-body sequential PET/MR system. In:
lEEEnuelear science symposium conferenee record. Ortande. 2009.
pp3508-3512
Lonsdale MN. BeyerT (20 W) Dual-rrodality PET/cr
irstrurnentatior-today and tomorrow. Eur J Radio! 73:452-6O
Punwani S, Taylor SA. Bainbridge A el al (2010) Pediatrie und
adoles-cent Iymphoma: comparison of whole-body STIR
half-FourierRARE MR imaging with un enhaneed PET/cr reference for
initialstaging. Radiology 255: 182-190
Schmidt GP. Reiser MF. Baur-Melnyk A (2009) Whole-body MRI
forthe staging and foJlow-up of patients with metastusis. Eur J
Rudiol70:393-400
Shao Y, Cherry SR. Farahani K et al (1997) Simultaneous PET and
MRirnaging. Phys Med Bio142: 1965-1970
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2PET/MRInstrumentationT. Beyer, O. Mawlawi, and H.H. Quick
ContentsIntroduction ...
PET/MR Design Concepts ....
MR-Compatiblc PET Detcctors
PET/MR Methodological Pilfallsand Tcchnological Challengcs
..
PET/MR Safety
~
Introduction
14Anato-metabolic Imaging
Summary and Conclusion ..
T. Beyer(BI)Center for Medical Physics and Biornedlcal
Engineering,General Hospital Vienna. Medical University Vienna.4L
Waehringer Guertel 18-20. 1090 Vienna. Austriae-mail:
[email protected]
O. MawlawiDepartment of Imaging Physics.MD Anderson Cancer
Center. Uuit 1352. Houston. TX 77030. USA
H.H. QuickInstitute of Medical Physics
(IMP),Friedrich-Alexnnder-University (FAU)
Erlangen-Nrnberg,Henkestr, 91. 91052 Erlangen. Germany
18 Most people require diagrostic tests during their lifetime
inorder to detect a suspected malignancy, plan a therapy
andfollow-up on a treatment. In almosr all of these cases
diag-nostic tests ertail a single imaging exarnination or a series
ofcomplementary imaging exarns, Nor-invasive irnaging iscentral to
personalized disease managernert and includesimaging technologies
such as Computcd Tomography (CT),Single Photon Emission Computed
Tomography (SPECT),Magnetic Resonance !maging (MR!). Ultrasound
(US) orPositron Emissior Tomography (PET).
Each of the above imaging tests yields a wealth of infor-mation
that can be separated generally into anatemical andrnetabolic
information. Anatemical information, such asobtained from CT or US.
is represented by a set of sub-mmresolution images that depiet
gross anatamy for organ andtissue delineation. Malignart disease is
typically detected onthese images by rneans of locally altered
image contrast orby ab normal deviatiors from standard human
aratorny. lt isimportant to note, that anatomical changes do not
necessarilyrelate LO the orset of rnalignan diseases. In other
words,maligrunt diseases are expressed as abnormal alterations
ofsignaling or metabolic pathways that may lead to
detectableanatomical changes. Therefore, anatomical imaging
aloneOlay rniss diseases frequently or diagnose diseases at
anadvanced stage only.
PET. as a representative of nuclear medicine imagingmethods. has
been shown to support accurate diagnosis ofmalignant disease [] as
well as providing essertial informa-tion for early diagnosis of
neurodegenerative diseases [2]and malfunctions of the
cardiovascular system [3]. However,over 90 % of all PET
examinations are performed for oncol-ogy indications. PET is based
on the lise of race amounts ofradioactively labeled biomolecules.
such as [18F]-FDG, afluorine-l S labeled analogue to the glucose
molecule, thatare injected into the patient whereby the
distribution of the
21
24
25
27
..~~ _ o. Ratib e al. (cds.). Atlas of PET/MR lnaging i
Ocoiogy,____ DOI 10.1007/978-3-641-31292-1_2. Springcr- Verlag
Berlin Heidelberg 2013
-
T. Beyer et aL.
p.n+e+v.~ e + range~"", -,;'-/ ~/"',/ ',~i N~: Ccincidence 0i!!i
"i r:l _~-----t:.J
np p
p P8nJpC!0
Sinograms
Reconstructed image
Fig. 2. Schernattcs of PET imaging: u biornoleeule is labeled
with apositron emilter (e.g .. IlIF, Tn - 109.8 min) and injccted
into the patients.The radioactive isoope labei decays by emining a
positron. which arni-hilutes with an electron from the surrounding
tissue. hus creating two
racer is followed by detecting the annihilation photonsresulring
from the ernission and annihilation of the positrons(Fig.2.1).
in most cases of malignant diseases early diagnosis is keyand,
therefore, imaging the anatamy of a patient may notsuffice in
rendering a correct and imely diagnosis. Thus, med-ical doctors
rypically employ a combination of imaging tech-niques during the
course ofdiagnosis and subsequent treatmentto monitor their
patients. Henceforth, both functional and ana-tomical information
are essential in state-of-the-art patientmanagement. An
appreciation for this type of combined infor-mation is best
illustraed with the introduction of the term"anato-rnetabolic
imaging' [4], in reference to an ideal imag-ing rnodality that
gathers both anatomical and functionalinformation, preferably
within the same examination.
Historically, medical devices to image either
anatomicalstructure or functional processes have developed along
same-what independent paths, The recognition that combiningimages
from different modalities can offer significant diagros-tic
advantages gaye rise to sophisticated software techriquesto
co-register (aka align, fuse, superimpose) structure andfunction
retrospectively (Fig. 2.2). The usefulness of combi n-ing
anaornical and functional planar images was evident tophysicians as
early as in the 1960s [5]. Sophisticated imagefusion software was
developed from the Iate 1980s onwards.
HO-CH,
Neutron delicientisotope
18F-FDG glucose
~/--;./. ,,",-~
..,,/7" /-'/~/
PET tomograph
annihilation photous ha are ernitted hack-to-back and detected
by aring of PET detectcrs. Image reconstruction then follows the
same prir-ciples as in CT (Courtesy of David W Townsend.
Singapore)
For relauvely rigid objects such as the brair, software can
suc-cessfully align images from MR, CT and PET, whereas inmore
flexible enviranments. such as the rest of the body, accu-rate
spatial alignment is difficult owing to the large number ofpossible
degrees of freedoru. Alternatives to software-besedfusion have now
becorne available through instrumentationthat combines two
complementary imaging modalities withina single system. an approach
that has since been termed hard-ware fusion. A cornbined, or
hybrid, tomograph such as PET/CT can acquire co-registered
structural and functional infor-mation within a single study. The
data are complementaryallowing cr to accurately localize functional
abnonnalitiesand PET to highlight areas of ab normal
metabolism.
The advantages of integrated, anato-rnetabolic imagingare
manifold [6]. A single imaging exarnination providescomprehensive
information on the state of a disease.Consequently. funetional
information is gathered and di s-played in an anatornical context.
Patients are invited for onlyone, instead of multiple exams. As
shown by several groups,the combination of complementary imaging
modalities canyield syergy effects for the acquisition and
processing ofimage data [7. 8]. And, firally, expers in radology
andnuclear medicine are forced to diseuss and integrate
theirknowledge in one report, which will perhaps be more
appre-ciared and considered a benefit in the years to come.
2 PET/MR Instrumentation
1960
1990
Semi-automated SW co-registration
2Oj~i.QI~ -~~C~.'1' _~. _ ...:;~'-'t .- . 1~i ~ ~_~
Ji.
Hardware-based fusionPET/MR
1980
Lightbox viewing and film
e.
2000
SPECT/CT
.~~~ . ~!~'~,r-~
;... - '-'':rr- ~.:.~,~~- ~~-~ ... '-..\_~~.
-r~'Hardware-based fusion: PET/CT, SPECT/CT
Fig. 2.2 The history of fusion imaging: from the 19605 to the
19905complementary image information was aligned rnanually and
later withthe srpport of computer-based algorithms.With the
introduction of Pr'>
PET/CT Imaging
PET imaging has been in elinical practice since the Iate1980s,
thus providing valuable information in addition to CTimaging in
cases where complementary diagnostic informa-tion was elinically
indicated, However, the lack of fine ana-tornical detail in PET
images may limit the Iocalization oflesions and permit only a poor
definition of lesion boundar-ies. This challenge was overcome by
combining high-resolution anatemical cr imaging with PET, thus,
providinga hardware combination for "anato-rretabolic" imaging
L9].The first proposal to combine PET with CT was made in theearly
1990s by Townserd, Nun and co-workers. The fore-most benefit of a
PET/CT hardware combination was theintrinsic alignment of
complementary image information,further supported by a elinical
need at the time. A secondaryberefit of this combination came with
the ability to use thecr irnages to derive the required PET
attenuation correctionfactors, one of the pre-requisites for
quantitative PET imag-
totype SPECTICT and PET/cr imaging in the 1990s and
PET/MRimaging systcms in the mid 2000s the field of hardware image
fusionwas changed drarratically
ing [IOJ. CT-based attenuation correction has now becornethe
standard in all PET/CT ornographs [ll] despite the factthat same
assumptions have to be made in order to transformthe attcruation
vaIues of human tissues at cr energies (e.g.effective CT energies
are on the order of 60-90 keV) toattenuation coefficients at the
PET energy of 511 keV [12,13]. Figure 2.3 illustrates the main
drivers for PET/CT:anato-rnetabolic alignment and CT-based
attenuationcorrection,
Following the introduction and validatior of the firstwhole-body
PET/CT prototype in 1998 [14] first cornmercialPET/CT concepts were
proposed as of 2001 leading to abreadth of 25 different elinical
PET/CT systems offered bysix vendors worldwide in 2006. Today, four
major vendorsoffer a range of whole-body PET/CT syserns with
greatlyimproved functionalities forboh, PETand CT [151. Table
2.1sumrnarizes the state-of-the-art PET/CT technology. In brief,all
PET/CT systems permit total-body imaging within a sin-gle
exarnination while using the available CT image
-
o
Functioral anatamyHigh functionl resolutionEarly datectlon
possible
PET = Emission
Topogram
Fig.2.3 PET and CT can be operared in closc spatial proximity
with-out cross-talk degradation of their respective perforrnarce
pararneters.(a) PET and CT images provide complementary diagnostic
irforma-
information for routine atteruarion and seatter correcrion ofthe
PET data [6]. Major technical advances include the incor-porntion
of ime-of-flight (TOF) PET acquisition mode [161,the extersion of
the axial field-of-view (FOV) of the PET[17] and the ineorporation
of system information, such as thevariability of the point spread
function across the field-of-view, into the reconstruction process
[17].
Time-of-flight-PET was first suggested in the Iate 1960sin order
to improve the signal-to-noise ratio (SNR) of thePET data [18]. In
essence. TOF-PET requires the measure-ment of the arrival time of
tWQ arnihilation photors arisingfrom a given annihilaton; which
helps localize the origin ofthe annihilation (i.e. the traeer)
beer. TOF-PET requiresrast scintillation detectors and advanced
detector electronics(see also seetion "MR-Compatible PET
Detectors").In human studies TOF-PET can help increase the SNR by
ataeter of 2. Today. still few studies are available that
demon-strate a significant diagnostic benefit in routine elinical
appli-cations [19, 20], but the options for trading a gain in
SNRinto reduced injected activities or into shorter ernission
seantimes are available taday.
Extending the axial FOV of a PET system comes at theexperse of
more PET detectors to be added in the axialdirecion. However. for a
given injected aciviy, more
T. Beyer et a'
~---
CT ;: Transmission
\1'S- -
CT Emission Attn-corr Emission
tion. (b) The use of the cr transmission images for the purpose
ofnciseless ntenuation correction of the emission data comes as a
second-ary benefit of PET/cr
annihilation photon can be detected. thus. increasing the
sys-tem sensitivity by 80 % for an additional 25 % axial cover-age.
This gain n sensitivity can be used for redueed emissionsean times
or activities injeeted. Despite the required increasein axial bed
position overlap, the number of contiguous bedpositiors required to
cover a given eo-axiaI imaging range isredueed in case of PET
imaging sysems with an extendedaxial FOV.
Parallax errors arising from depth-of-interaction effeetscause
the spatial resolution of the PET to be a variart of thespatial
location of the annihilation. if the spatial variation ofthe
poin-spread-function (PSF) is known apriori. for exam-ple, by means
of standardized measurements, it can beincluded in the
reconstruction algorithm [17, 21]. The recon-struction process
becomes computationally demanding buthelps improve the spatial
resolution and renders the varia-tions of the PSF in the images
uniform across the field-of-
view,Over the years, the above advances have helped improve
the qualiy and reproducibility of PET and PET/CT data(Fig. 2.4)
and suppo1 a routine examination time for a stan-dard whole-body
FDG-PET/CT study of 15 min. or less, asignificant advantage when
eompared to PET/CT irnaging
from a deeade ago.
2 PET/MR Instrumentation11
Table 2.1 State-of-the-art PET/CT imaging systems GE Healthcare.
Philips Healthcare. Mediso and Siemens Healthcare (from left LO
riglt). Thefigure shows key parameters and performance measures of
the PET/Cl series
Discovery VCT Ingenuity TF AnyScan Biograph mCT
;;;
~
CT: 16-128 slices CT: 16-128 slices 16-sliceCT CT: 20-128
70 cm patient port 70 cm (85 cm) patient port 70 cm diameter
patient port 78 cm patient port
250 kg table weight Hmit 215 kg tabla weight limit 250 kg table
weight limit 250 kg tabe weight limit
170 cm co-sean range 190 cm co-sean range 360 cm co-sean range
170 cm co-sean range
24 rings of LYSO(Ce) 44 rings of LYSO(Ce) 24 rings of LYSO(Ce)
52 rings of LSO (Ce) crystals
4.2 x 6.3 x 25 mm' 4.0 x 4.0 x 22 mm' 3.9 x 3.9 x 20 mm' 4.0 x
4.0 x 20 mm3
Time-of-flight Time-offlight Time-of-flight
15.1 cm axial FOV 18 cm axial eoverage 23 em axial eoverage 21.6
cm axial eoverage
70 cm transaxial FOV 67 cm transextat FOV 55 cm transaxial FOV
70 cm trarsaxial FOV
PET resolution model PET resolution model PET resolution
model
In-plane resolution: 4.9 mm In-plane resolution. 4.7 mm In-plane
resolution: 4.1 mm In-plane resolution: 4.4 mm
Axial resolution: 5.6 mm Axial resotution. 4.7 mm Axial
resolution: 4.2 mm Axial resolution: 4.4 mm
3D Sensitivity: 7.0 cps/kBq 3D Sensitivity: 7.0 cpsll
-
T. Beyer et al.12
Fig.2.4 Corenal (top) and transaxial (bottom) view of an
whole-body[ISFI-FDG-PET image of a putient with n BMI of 35
acquired in30-node with septa retracted and reconsructed using:
(~L)30 filteredback-projection algorithm with reprojecticn
(30-FBRP, 7 mm Gauss).(h) elinical reconsrruction using FORE
rebinning+20 OSEM (8 sub-
sets. 3 uerations: 5 mm filter), (c) 30 Ordinary Poisson
(QP)-OSEMwith PSF reconstruction (14 subsets.2 irerarions: no
smoothing), and(d) 3D OP-OSEM wih both PSF and Tine-of-Flight (TOF)
reconsruc-lion (14 subsets. 2 iterations. no smocthing) (Case
courtesy of DWTowrsend, Singapore)
Fig.2.5 Expectations for PET/MR in the context of the
existingexperiences with PET/CT forpauent imaging
C'~~~(Ul~~~U
PET/CT PETlMR
pro
High-resolution anatamy Best passible, intrinsic ca-registratian
Quasi-simultaneaus Jmaging Noiseless/fast attenuation carreetion
Fast whole body imaging Integrated report
High saft tlssue contrast through MR Simulataneous imaging
possible Less ionizing radiation (MR=O) MR upgrade=New MA
sequences
con
MRcompatible PET detector MRbased attenuatien ccrrectlon PET/MR
design restrictions Patient acceptance Clinical and research
apoucattons
Patient exposure from CT Local, motion-induced misalignment Only
quasl-simultaneous scanning Hardware Upgrades via fork-Iift
Reimbursement for PETonclear
imaging togetler, with the added potential of MR-based
motioncorrection of the PET data, significantly reduced patiert
expo-sure and a increased soft issue contrast through the use of
MRinstead of cr, wherever c1inically indicated.
Soft tissue enhaneement in MR (versus cr) may benefitthe imaging
of pediatric patients where normally little fattytissues are
present (Fig. 2.6). as well as for studying patientsfor indieations
related to the brain, parenchymal organs or the
2 PET/MR Instrumentation
Fig. 2.6 Slde-by-side comparison of cr. MR and PET images of
apatient with previously irradiated fibrosarcoma. The tumcur is
poorlyvisualised on cr but the MRI shows a residual mass.The PET
sbowsresidal moderate FDG-avidity, and resection confirmed residual
viable
rnusculoskeletal system. In addition to mueh improved softtissue
contrast MR is a versatile imaging rnodality sine e it pro-vides
addirional measures of physiologic and metabolic char-aeteristies
of human tissue [251. MRI goes beyond plainanatonucal imaging by
offering a multitude of endogenouscontrasts and a high capability
of differentiating soft tissues, aswell as many exogenous contrast
rnedia ranging from gadolin-ium-based agents to highly specified
cellular markers [261
MR specroscopy (MRS), for exarnple, can be used to dis-sect the
rnolecular composition of tissues by applying seIee-tive
radiofrequency exeiration pulses [271. Functionalprocesses in
living subjects can also be studied via diffusion-weighted (DWJ)
MR! [28]. Here, a spatially and tenporallyvanant magnetic field,
generated by different magnetic fieldgradients in allthree spatial
directions. is used to rnap phasedifferenccs in the MR! signal that
are caused by diffusirgnolecules. DW!-MRI has potential elinical
applicationsranging from diagrosing ischemia in early stroke
diagnos-ties, cancer, multiple selerosis, or Alzheirner's disease
togeneral fiber traeking via diffusion tensor imaging
(DT!)[26,29,30], and it is not restricted to the brain [3IJ. In
addi-tion, funetional MR! (fMRI) studies can be performed dur-ing
the same exarnination. Functional MRI (fMRl) studiesare frequently
based on the BOLD (blood oxygen leveldependent) effect [32]. This
effect deseribes the fact that thernagneic properties of oxygenated
and deoxygenatedhemoglobin in the blood are different and.
therefore, produce
13
,
tumour. Lack of soft tissue contrnst. particularly lack of fa in
childrencompromises anatemical evaluation on cr compared to MRI
(Courtesyof Rod Hicks. Peter MacCal1um Caneer Cenre, Melbourne
Australia)
different signals when imaged with T2~-sensitive MRlsequences.
The BOLD effeet alsa has certain applications incancer irnaging,
such as to study tumor angiogenesis, turnoraxygenation and brain
activation in eloqueru areas prior tosurgical resection.
Any of the image information above can be acquired andpresented
in any directian in space. thus rendering re-orier-tatior of image
information in MR sirrilar to a "virtual tilt",that is available in
CT-only in directions perpendicular LO themain scanner axis, and
that are not available in PET/CTimaging.
Similar LO CT and PET, MRI has become a whole-bodyimaging
rnodality tharks, for example, to the advent of par-allel imaging
techniques and alitheir derivalives [33-35] andrhanks LO new
whole-body imaging strategies [36, 37]. Imageacquisition times have
been shortened, thus allowing fastsingle-contrast MR whole-body
eoverage from 30 s [36]ranging to muli-contrast. multi-station
whole-body MRIexarrinations to be acquired with high spatial
resolution inless than i h. lnitial results show that whole-body
MRI is apromising modality in oneology. especiaIly for the
deteetionof metastases and hematologic malignancies.
Therefore, MR! holds a great potential in replacing CT asthe
complementary rnodality to PET in dual-modality tomo-graphs for
selected indications where MR outperforms CTalready. In theory. MRI
seems a perfeet anatomical comple-ment to PET.
-
14
PET/MR Design Concepts
Following the successful adopion of PET/CT in elinicalrouine and
the ongoing efforts toward s combining PETand MR! for pre-clinical
research applications [241,industry has quickly adopted the idea of
combining PETand MR! for human studies. Figure 2.7 summarizes
themain approaches towards PET/MR hardware fusion. Inessence. three
different design concepts have been pro-posed: separate PET/CT and
MR! systems operared inadjacert rooms (a), PET and MR! sysems
arranged in thedirectian of the main scanner axis with a patient
handlingsystem mounted in between (b) and a fully integratedPET/MRI
system (c).
PET/CT-MR Shuttle System
GE Healthcare proposed a straightforward design in Iate2010.
This design is based on a combination of a dual-modality,
whole-body TOF-PET/CT and a 3 T MR systemthat are operared in
adjacent rooms; patients are shuttledfrom one system to the other
without getting off the bed[38]. This approach substitutes the
challenges of hardwareintegration for immense logisticn\ challerges
n timingaccess to the two systems while minimizing patient
rnotionin between examinations. The advantage of this rather
sim-plistic approach to PET/MR is that it is based on
existingimaging technologies without significnnt changes to
heirhardware components. Patients urdergo a PET/CT studyleveraging
the bercfits of time-of-flight PET as discussedbefore. Following
the PET/CT examination patients arethen Iifted on a mobile
docking-table system and shuttledto the MR system where a
loco-regional or whole-bodyMR study is performed depending on the
elinical indica-tion. Figure 2.8 illusttates a elinical ease from
the com-bined use of PET/CT and MRI Llsing the PET/CT/MR
system.While this design is stili available as prootype
technol-
ogy only, it has been argued alsa as the most
cost-effcctivecompared to fully integrated PET/MR based on
workflowaspects and machine utilization [39], both of which
aresite- and operatiors dependent. Therefore, in pracucethe
elinical and cost efficacy of the separate PET/CT/MR design opion
(Fig. 2.7a) wou ld be affected by vari-ous workflow and
installation requirements. For exarnple,both systerns need to be
installed next to each other andoperared within a combined
scheduling system. Anydeviation from standard protocols would
entail exterdedwaiting times with the patients Iying on the shuttle
sys-tem until the next exam can cornmcnce. Also, two or eventhree
shule systerns are required to facilitate a
seamless,high-throughput workflow. On the upside this approachdoes
ensure proper attenuation and seatter correction ofthe PET data
based on the available CT information. In
T. Beyer et aL.
Fig.2.7 Design concepts for PETlrvIR: PET/CT and MRI
tornographsare operated in adjacen rooms and intertirked with a
mobile shuttlesystem Ca). a co-planar PET/MR with a whole-body PET
and MR oper-ated in close proximity and a combined table platform
(b). and a fuUyintegruted PET/MR wirh MR-compatiblc PET deteetion
system slip-fitinto the MR (c) i'
turn, the examination time is likely to be the longest ofall
PET/MR designs and patient convenience is limitedby the
repositioning in MR or PET/CT using the shule
system.
Co-planar PET/MR
Philips Healthcare proposed a slightly more integratedapproach
to PETlMR in 2010 [40J. They also presented thefirst comrnercially
available PETlMR system for elinical usecalled the Philips
Ingenuity TF PET/MR!. The system(Fig. 2.7b) is based on a co-planar
design concept that inte-grates a whole-body time-of-ftight (TOF)
PET system and anAchieva 3 T X-series MR system. Both components
arejoined by a rotating table platform mounted in between [41].
The PET detector and electronics system is based on avail-able
Philips PET/CT technology. However, given the proxim-ity of the
PETand MR system (about4 m) some modificationswere required to
ensure MR-compatibility of the PET system.These modifications
include the addition of bulk magneticshielding of the PET to reduce
fringe magnetic fields, the useof higher permeability shields of
the photomultiplier tubes
2 PET/MR Instrumentation 15
UniversityHospitalZurichii-
'..:1.. \
o.. ...,
FDGPET/CTFDG-PET
Fig. 2.8 Patiert with large left lung lesion undergoing
wholc-bodyFDG.PET/CT and whole-body MR! on the separate PETeTIMR
sys-tem (Fig. 2.7a). From left lo right; FDG~PET following CT-based
atten-uanon correction (CT~AC). PET af ter TAC fused with
whole-body
(PMT) inside the PET gantry and the rotation of the cathodesof
the PMTs. Further, power and signal cables penetratingthe room
walls need to be filtered through specially designedradiofrequency
(RF) penetratian panels to preven extraneouselectrom:.gnetic
radiation to enter the scanner room and PETacquisition electronics
are enclosed in an RF tight cabinet.These and other modificalions
are diseussed in more deail in[411. The authors show that despite
the rnodificatiorsto the PET/M RI system components the performance
of nei-ther the PET nor of the MR is degraded, and that both
systemscan be operared in close spaial proxirniy.
Figure 2.9 illustrates a total-body imaging exarninationfrom the
co-planar PETIlVIR system. While this design con-ce pt nay be
regarded as a step closer towards integraredPETlMR (compared to
sequential imagirg, Fig. 2.7a) itoffers sequential PET and MR!
imaging with delays that areon the order of those in PET/CT and in
sequential PET/CT-MR imaging [42]. It could be argued that
co-registrationof PET and MR information is slightly better and.
perhapsmore reproducible, in both modalities compared to the
shut-tIe system in Fig. 2.7a. since patierts are not
relocatedbetween rooms and repositioned using a mobile patiert
han-dling system. However. no study to date has been able toverify
this. Unlike with the separate PET/CT-MR system.one modality is
idiing during co-planar PET/MR imaging,which may be argued to be
less cost-effective. However, oneshould keep in mind that today few
indications are c1earlydefined as key indications for PET/MR, and,
therefore.throughput is likely not an issue for the time being. The
co-planar PET/MR system offers full MR-flexibility and TOF-PET
functionality. Unlike with the separate design. notransmission
source is available, hus requiring MR-bascd
___.- - attenuation correction methods (see below).
.,
MRI FDG-PET-MRI
cr. complementary WB-MR and retrospectively aligned and
fuscdPETUCf)-MR (Data courtcsy of Patdek veit-Haibach. MD,
UniversityHospital Zurich)
Integrated PET/MR
The first PET/MR design for human use was presented asearly as
in 2006, representing also the most challengingdesign concept (Fig.
2.7c) [43]. This PETlMR prototype sys-tem (BrainPET, Siemens
Healthcare) was intended for brainimaging onlyand considered a
proof-of-concept for a fullyirtegrated PET/MR. The BrainPET system
was based on aPET detector ring designed as an insert to a 3T
whole-bodyMR scanner (Magentom Trio, Siemens Healthcare
Sector.Erlangen. Germary) with the novely being theMR-compatible
PET detection system that was integratedinto the MR system. Here.
the PMT were replaced byAvalanche photodiodes (APO), which have
been shown tooperate in magnetic fields of up to 7 T [44J (see also
seetion"MR-Compatible PET Detectors"). Therefore, in this designLSO
(lutetium oxyorthosilicate)-based derector blocks, com-prising of a
12 x 12 matrix of2.5 x2.5 x20 mm' crystals weredirectly coupled to
a compact 3 x 3 APO array. With this sys-tem PET and MRI cover an
active co-axial FOY of 19.3 cmsirnultaneously, The point source
sensitivity of the PET sys-tem measured with aline source in air w
as 5.6 % and thespatial resolution was 2. I mm at the cerre of the
FOY. Nodegradation of the MR images was observed due to the
presence of the PET detectors and no detrimental effeet on
theperformanee of the PET detectors was observed for a nurn-ber of
standard MR pulse sequenees [45]. Since 2006 theBrainPET was
installed at 4 sites worldw ide, with one siteoperating the PET
insert inside a 9.4 T MR! as welL. Sornepreliminary elinical
research data are deseribed in [46-481.Looked upon retrospectively,
the elinical test phase of theBrainPET helped pave the road towards
whole-body PET/MR, the advanced development of MR-based
attenuation
-
16T. Beyer et a'
Fig.2.9 29-y/o Iernale paticnt with Maffucci syndrome diagnosed
inher childhood, This disease is sporadic wilh multiple
enchondromasand hemangiomas. An [18FI-FOG-PET/CT total body study
was per-formed for staging. Subsequent total-body PET/NIR. using
the same
correction and, perhaps most importantly, an improved
COI11-municatior and closer collaboration of radiologists,
nuclearmedicine physicians and physicists.
Based on the aforementioned positive BrainPET experi-ences a
further step towards the integrared design corcept(Fig. 2.7c) was
suggested in Iate 2010. Then, the first who le-body, integrated
PETfMRI system (Biograph mMR, SiemensHealhcare) was proposed. Each
PET detector block consistsof an 8 x 8 matrix of LSO crystals
coupled to a 3 x 3 APD-array. The transaxial FOV of the MR is SO
cm. whereas theaxial FOV is 45 cm. The PET subsystem consists of 8
ringsof 56 blocks with tm axial FOV of 25.8 cm and ring diameterof
65.6 cm. Both. the extended axial FOV of the PET and theredueed
ring diameter help inerease the sensitivity of thePET insert. which
in tum could be leveraged. for exarnple.for shorter emission sean
times or reduced injected PETactivities. Thus, tle lack of
TOF-capability in APD-bascdPET systerns (see Chap. 3) can be
compensated for. in the-ory, by bringing the PET deectors eloser to
the centre of theFOV and by extending the axial coverage. A
detailed descrip-tion of the system together with a performance
characeristic
FDG injection. rrore elearly presents bone involverrcnt and is
preferredbecause of the need of multiple follow-up exarninations
(Data courtesyof Osman Ranb. MD, University Hospital Geneva)
can be found in [491. On the downside of the eloser integra-tion
the integrated PET/MR system, the bore drameter isreduced to 60 cm.
which - for the moment - is the reversetrend of PET/CT and MR-only
instrumentation with ganryand borc diameters of up to 80 and 70 cm.
respectively,Inerensed ganry and bore diarneters help improve
patiencomfort and compliance and, in addition. leave room
forimage-guided interventions, if needed.
Perhaps most irnportantly, the integrated PET~IR designcorcept
allows for simultaneous data acquisition. However,simultaneity of
complementary volumetric data acquisitionis assured only for a
selected MR sequence and emissiondata that are acquired for the
duratior of that specific MRsequence. Nonetheless, simultaneous
PETfMR is argued toimprove the diagnostic accuracy of combined
PET/MR oversequential imaging (Fig. 2.7a, b). Figure 2.10
illustrates acase from the Biograph mMR system with very good
spatialalignment of PET and MR images in the abdomen.
While the benefit from improved spauo-ternporal align-ment is
imrnanent to the PETIMR images from integratedPETfMR it is not elem
as to how rnuch it is required for elinical
2 PET/MR Instrumentation
Fig.2.10 61-y/o ferrale with known squamous cell carcinoma of
thelung undergoing [18F}-FDG-PET/MR imaging on an
integratedBiograph mMR PETINIR system. (a) lrcreased PET tracer
activity syn-onymous of disseminated disease is depicted in the
bronchia! carci-
rouine. Further, PET and MRdata are simultaneously acquiredonly
for a limited period of time or for a selected region. orvoxel in
Is extreme. Without a doubt, the closer alignment ofPET and MR data
in both, an anatemical framework and overvarious imaging times will
help in elinical researeh, such aswhen comparing perfusion studies
with II SAl-water and ane-rial spin labeling (ASL) in MR [24J.
Also, using integratedPETINlR imaging for shortening combined
examinauon timesover those n sequertial and co-planar designs is
preferred forthe well-being of patients with acute diseases,
pediatric patientsrequiring sedation and patierts with
neurodegenerative dis-eases. Finally, since MR-based rnotion
detectior is conceiv-able during simultaneous PET/MR irnaging, such
MR-derivedmotion vector can potentially be used to eorrect for
motion-indueed blurring of the PET emission data [50, SI J.
As with the co-planar design. the integrated PET/MRdesign does
not allow for separate transmission imaging and,therefore,
PET-based attenuation data must be derived fromthe available MR
information. Thus, anormal workftowstarts with the simulaneous
acquisition of emission data anda dedicated MR sequence for the
purpose of deriving attenu-ation data. As soon as the short
MR-attenuation sequenee iscomplete, addirional diagnostic MR
sequenees can be
17
'"
noma. frenral lobe metastasis. pancreas and in secondary.
metastaticcolcrectal eaneer. Coronal whole-body Tl-weighted MR
image (b).attenuation corrected PET (d), and PET/MR image (i) and
correspond-ing axiul images through the bronehial carcinoma are
shown (c. e. g)
acquired for the remainder of the pre-defined emission
sean.Alternatively, the PET emission data can be acquired in
list-mode format and reframed af ter finishing the MR sean,
Table 2.2 provides an overview of currently availablePETiMR
sysems, All systems support the acquisition ofwhole-body, ifnot
total-body, examinatiors. These first PET/MR design concepts vary
more widely than the first conceptsfor PET/CT. This variation can
be explained by the cornplexphysical and more dernanding echnical
requirements for afull integration of PET and MR imaging systems,
comparedto those from a PET/CT integration.
The foremos diffcrence betwcen the PETiMR systems isthe type of
PET detector. Integrated PETiMR imagingrequires a novel PET-based
detection system, which will beexplained in more detail in Chap. 3.
APO-PET does not sup-port TOF-based acquisitiors due to the
insufficient timingresolution of the APO, thus, only two PETfMR
designs offerTOF-capabilities (Table 2.2). Major differences are
alsa seenin the patient table design. which has subsequenteffects
on thehandling of the patients and worktlow. Both, the co-planarand
the integrared PETiMR require the use of the MR imagesfor human
soft tissue attenuation correction, which today isperhaps the
biggest challenge for combined PET/MR in the
-
18T. Beyer et aL.
Table 2.2 State-of-the-art PET/MR imaging systems by GE
Healthcare. Philips Healthcare and Siemens Healthcare (from left to
righr). Note. asof 2012 the Philips and Siemens system were
FDA-approvcd and ccmmercially available. The figure shows key
parameters and performunce
measures of the various PETlMR series
Discovery PET/CT-MR Ingenuily TF Biograph mMR
2 PET/MR Instrumentation 19
-. , .._,"';:ik. ..- .~\ PMT
Scintiltator
aB> OT
b Avalanche photadiedes
APD-based
PET detector
ot
MROiscovery MR 750w
70 cm bore diameter50 x 50 x 45 cm FOV
t 6 (32) receive channels0.5 ppm field homogeneily
PETDiscovery PET/CT 690
4.2 x 6.3 x 25 mm3 LYSO(Ce)81 cm detector ring diameter
Time-of-flightPET15.7 cm axial coverage
70 cm bore diameter
Patient handling system
Shuttle and docking system159 kg patienlload
170 cm co-sean range
CT-based AC
MRAchieva 3T TX
60 cm bore diameter50 x 50 x 45 cm FOV
16 (32) receive channels
0.5 ppm field homogeneityPET
MR3T (nol Verio-based)60 cm bore diameter50 x 50 x 45 cm FOV
18/32 reeeive ehannels
0.1 ppm field homogeneityPET
4.0 x 4.0 x 20 mm' LSO66 em ring diameter
No Time-of-flightoption25.8 em axial eoverage
60 em bore diameterPatient handling system
Inlegrated table platform200 kg patient load
140 cm co-sean range
MR-based AC
In-plan e resolution: 4.9 mm
Axial resolution: 5.6 mm30 Sensitivity: 7.0 eps/kBqPeak NECR:
130 kcpsCoinedenee 4.9 ns
TOF resolution: 549 ps
In-plane resolution: 4.9 mmAxial resolution: 4.9 mm
3D Sensitivity: 6.4 cpslkBq (NEMA)Peak NECR: >91kcps @
16kBqiml
Comeldence : 3.8 nsTOF resolution. 535 ps
In-plane resolution: 4.5 mm
Axial resolution. 4.5 mm3D Sensitivily: 13.2 cpslkBq
Pea k NECR: 175 kcpsCoincidenee : 5.9 ns
NoTOFFiq, 2.11 (a) Convennonat PET detectcrs using
photorrultiplier tubes(PM7) do not work inside a magnetic field.
This is illustrated by thescintillatcr position profile that is
skcwed already from fringe fieldsfrom a horseshoe magnet pluced
next to the PET deteetorIPMR(Courtesy Bernd Pichler. University of
Tbingen). New PET detectors
Compaet
High quantum effideney
Law bias voltage
Magnetic field nsensitiveLower gain
Limited time resolution
c SiPM or G-APD
SiPM-basedPET deteclor
4.0 x 4.0 x 22 mm' LYSO(Ce)90 cm deteetor ring diameter
Time-of-flightPET18 cm axial coverage
70 cm bore diameter
Patient handling system
Turning table platform200 kg patient load
190 cm co-sean range
MR-based AC
Highgain
Low bias voltage
Compact
Magnetic field insensitive
Very fas and TOF- compatible
Low cost CMOS process
Low quantum efflciency
Low photon detection efficieney
(b) based on avalanche photodicdes (APD) can be rnade more
compactand have been shown to perform in rnagnetic fields up to
9.4T. Recemdevelopments indicare further improvcments for
MR-compatiblc PETdetectors based on SiPM. a type of Geiger-APOs
(c)
light of c1inieally adopted PET/CT imaging when
absolutequantification of PET data is eonsidered.
MR-Compatible PET Detectors
Cross-talk effeets between PET and MRI may oceur wheninserting a
eonventional PET derector and associated elec-troric components
into an existing MR system. This rnayrelare to disruptions of the
PET sigral cascade as well as todegraded MR maging. The possible
interactions betweenPET and MR signal generation are manifold. A
perfeet tech-nical integration of bat h modalities requires the MR
with itselectronagnetic environment not to disturb the sensitive
PETsignals. This encornpasses the strong static and honoge-neo us
magnetic field for spin a!ignment (in the range of sev-
eral Tesla), the strong spatially (mT/m) and temporally(mT/m/ms)
varying electrornagneic gradient fields for
spatial signnl localization, as well as the pulsed
rudiofre-queney (RF) transmit and receive fields for spin
exeirationand RF signal reception (MHz range). On the other
hand,PET system components must not intertere with any of theabove
listed eleetromagnetic fields of the MR system.
Consequently, for a fully-integrated PET/MR system. allPET
eleetronics must be RF shielded in order not to disturbthe highly
sensitive RF sigrals detected by the MR compo-nents. When shielding
the PET components that are locatedclose to the MR gradient coils.
the RF shielding has to bedesigned such that the strong time
variaut gradient pulses donot produce unwanted Eddy currents in the
shielding, whichrnay have a negaive effect on the gradient
linearity, poten-tially Icading to image distortions [521
Given the design of standard PET detectors based on
pho-tomultiplier tubes (PMTs). a PET/MR corfiguraion is obvi-ously
technically more challenging than the combination ofPET and CT
becausc phototubes are sensitiye even to low
magnetie fields (Fig. 2.11). Therefore, early developers
ofPET/lvIR co nce pts. such as Hammer and co-workers, pro-posed to
place only the PET scintillator inside the MR and touse light
guides to channel the scintillation light from thedetector to the
PMT situated outside the prirnary magnericfield of the MR system
[53]. This idea was advanced furtherby other groups. as discussed
by Wehrl in arecent review ofthe origins of PET/MR [24].
In order to provide PET perfornanee in PET/MR that issimilar to
PET performance in PET/CT. any MR-compatiblePET detector mu st
support accurate 30 positioning and veryfast timing information at
no cost of volurne sersitivity. This.n turn, calls for combinations
of scintillators with novel,
MR-compatible photodeteetors of high granularity such
asAvalanche Phoodiodes (APO) and Silicon Photornultipliers(SiPM)
[54].
Avalanche Photodiodes (APO]
Avalanche photodiodes (APO) are semiconduetor devicesthat
transform deteeted light into an electrical signal follow-ing the
same principles as ordinary photodiodes, However.nlike ordirary
phoodiodes, APD's operate exclusively athigh electric fields. When
an electron-hole pair is generatedby photon absorption, the
electron (or the hole) accelerateand gain sufficient energy from
the high electric field beforeit collides with the erystallattice
and generates another elec-tran-hale pa ir while losing so me of
its kineric energy in theprocess (Fig. 2.11b). This process is
known as inpaet ioriza-tion. The original as well as the secondary
electron (or hele)can the n accelerate again under the inftuenee of
the highelectric field and create more electron hole pairs.
Thisprocess ereares an avalanche of electron hole pairs - hence
-
201. Beyer et aL.
the name avalanche photodiodes. The rate at which electron-hale
pairs are generated by impact ionization is balanced bythe rate at
which they exit the high-field region and are eel-lected. If the
magnitude of the electric field (reverse-biasvoltage) is below a
value known as the breakdown voltage,the rate of collection exceeds
that of electror hale erentionand causes the population of
electrors and holes to deelineand eventually stops.
The number of created electron-hole pairs, referred to
asinternal gain, is typically in the ran ge of 10'_10' and
deper-dent on the electric field strength (reverse bias
voltage).Because the average number of created electron-hole pair
isstrictly proportional to the incident Iight photons, this modeof
operation is known as linear mode.
Unlike amplification in PMTs. the internal gain of APDsis
characterized by fluctuations due to the statistical nature
ofimpact ionization. These gain flctuations produce excessnoise,
which increases as the internal gain increases by rais-ing the
reverse bias. Other factors that affect the performanceof APO
include temperature. doping. as well as diode rnate-rial
properties. In addition. APDs are characterized by a rela-tively
long timing resolution (FWHM> 1,000 ps), whichlimits their use
in TOF PET systems. Bccause of the se fac-ters, it is desirable to
use APDs at moderate reverse bias volt-age and temperature to
ensure their stable operation.
On the other hand, APDs are characrerized by high
quantumefficiency (QE - number of elecrron or holes created per
num-ber of incidem scintillation photons) particularly at the
wave-lengths of PET scintillaion detectors. APDs are alsa immline(o
after-pulsing, which are spurious pulses gererated
fromelectron-holes being rapped by crysal defects and releasedatter
a certain delay time, thus, confounding the detection pro-cess.
Most impotantly and conrary to PMTs, APDs areirnmune to stationary
and varying magnetic fields, thus, render-ing them suiable for
PET/MR systems. APDs typically have amaximum size limited to about
i cm'. due to the difficulty ofmanufccruring large area
scmicorductor devices. however, theeost of rnanufacturing APDs is
relatively law.
SiIicone Photomultipliers (SiPM)
A promising development in photodetectior for PET im:.g-ing is
the introduction of Geiger mode avaianche photodiodes(G-APD. Fig.
2.llc), commonly referred to as silicon photo-multiplier (SiPM).
This is a novel type of photodetector thatis about to reuch a
performance level that offers significantimprovement over APD-based
PET.
A SiPM is an APD operated with areverse bias voltageabove the
breakdown voltage (-50-60 V above breakdownvoltage). In this case,
the electron hale pairs generated byphoton absorption will multiply
by impact ionization fasterthan they can be extracted, hus,
resulring in an exponential
growth of electron-hole pairs and their associated
photocur-rert. This process is known as Geiger discharge. The
currentflow produced by the Geiger discharge is large and results
ina large signal gain (more than 10'). Following a Geiger
dis-charge, the SiPM is reset by dropping (quenching) the volt-age
across the photodiode below the breakdown voltage.This will reduce
the number of created electron hale pairsand eventually stop the
Geiger discharge. The discharge-and-reset cycle is known as the
Geiger mode of operation ofthe photodiode. The turn-on transient of
the current dischargeis cornparatively fast, with several
picoseconds while thetum-off transient through quenching is mostly
dependent onthe SiPM size and is on the order of 100 ns. Quenching
canbe achieved using active or passive techniques although forhigh
counting capabilities, active quenching is preferred.
One important application of SiPMs is the ir abiliy tocount
photons, which could be used to determine the energyof the incident
annihilation photon on a scintillator in a PETsystem. However, a
single SiPM eell has a linnitation in thatit is essentially either
on or off. It eannot distinguish betweera single and multiple
photons that arrive simultaneously. Oneonly knows that the APO was
triggered. This limitation,however, is overcorne when us ing an
array of SiPM cells thatare connected in parallel. In this case the
output of the SiPMarray is the sum of the output of each SiPM cell
(pixel) in thearray. For exarnple, when the photon ftux is lawand
photonsarrive at a time interval longer than the recovery time of
apixel, the array will output pulses that equate lo a single
pho-toelectron. The gererated pulses are then converted to
digialpulses and courted. However. when the photon flux is highor
the photons arrive in short pulses (pulse width less thanthe
recovery time of the SiPM), the pixel outputs will add upto the
equivalen number of incident photons. In this ense,the SiPM array
behaves in a pseudo-analog marner, becauseit can measure the
incident number of photons per pulse,which is not possible with
single photon counting SiPMs.
An imprtnnt feature of SiPMs is their immunity to excessnoise.
This is primarily due to the fxed number of electron-hale pairs
produeed in Geiger mode, which is not defined bythe statistics of
the impact ionization process as in APDs.Anather irnportant feature
of SiPMs is their relatively fastrise time, and short time jitter
(FWHM = 0.1 ns) defined asthe statistical variation of time
interval betweer the photonarrival and the resulting electrical
sigral from the SiPM -thus. supporting their use in TOF-PET
tomographs.Furtherrrore, the performance of SiPMs (like APDs)
isimmune to the effects of stationary and temporally
varyingmagnetic fields which allows their use in PETlMR
systems.
On the other hard, SiPMs have a relatively law photondetection
efficiency (POE). due to their lAWQE for scintilla-tion light from
PET detectors (40 % at 420 nn). In addition.SiPMs are characterized
by high dark count rae, high crosstalk and af ter pulsing as well
as a strong ternperature and
2 PET/MR Instrumentation 21
PMT . -- APD ---- iPMActive area (mm 2) t-2,ooocm' 1-100 mm'
I-LO mm'
Gain HJ'-IO' lO' 10'_10'
Rise time
-
22T. Beyer et a'
~.?"If;;~ '":~......@ ,~-@.~ ~~ ~ '"X), re; "'. i XPET emission
(30 min) PET transmission (15 min) Attenuation-corrected
PET emission imagea
AUenuation correction tactar
i; i e -oL ~(X, E PET)dxo i .
! Attenuaton
i True PET Signal
Tissue attenuatloruevdensity) in (HUIb
Aif -1000
Lungs -850 ... -250
Water OSaft tissue 20 ... 300
Bone 300 .. 2000 CT transmission (1 min)
,?.-i
Proton density in tissueT1 ,T2,T2w relaxation parameters
cMR,AC-sequence (2 min)
Fig. 2. 2 Cballenges of MR-based auenuation correction. (a) in
PETattenuancn correction facors can be calculated from separate
PETtransmission measurements. which take a relatively long time but
pro-vide attenuation values at s keV (b) in PET/cr Cl-based
attenuationvalues. representing a measure of the electron-density.
can be used to
estimate PET attenuaticn coefficients. (c) In PETlMR no measure
ofeleetten density is available and tissue appearance on MR and
crimages is markedly different for air and bone. Therefore. no
direct rnea-surement is available for MR-based attenuation
coefficients
for reconstruction of fat-only. water-only and fat-waterimages,
and results in tissue segrnentation of air, fat, muscle,and lungs
[6IJ. Bone is not accounted for in this approach.Initial results in
elinical pilot studies have shown that thisapproach works reliably
and provides results that are cornpa-rable to corrected images form
PET/CT in the same irdivid- _ual, However, further studies are
needed to assess the impactof ignoring bone and the overall
aceuraey of MR-based ACmethods on PET quantification.
This relates to the patient table, transmit and receive
radio-frequency (RF) coils as well as positioning aids, The fact
thatthe RF eoils are located inside the FOV of the PET system(Fig.
2.7b, c) is a ch alien ge and has only started to beaddressed. For
brain seans, the head coil is rigid and its atter-uatior values can
be estinated from a refererce CT-bascdauenuation map, Subsequently
for any PET/MR studyonlythe relative position of the head coil
inside the PETIlvIR sys-tem would be required, Additicnal work has
been directedtowards reducing the amount of attenuating materials
in N1Rcoils used in PET/CT as exernplified in a rnodified brain
coilfor integrared PETIlvIR irnaging [62, 63].
For extru-cranial exarninations the situation is moredemanding.
MR surface coils are required to achieve
optimalsignal-to-noise-raio (SNR) and high quality MR
irnages,Surface RF coils may contain elastie components and
hencetheir individual position on the patient cannot easily be
The Effect of MR Radiofrequency (RF)Coils on MR-AC
In addition to the general transformatian of suitable rvlRimage
information of paien tissues. other. hardware-relatedattenuators mu
st be considered during the transformution.
2 PET/MR lnstrumentation 23
Fig.2.13 Ccntributors toattenuation and image distortionsin
PET/MR. Topics marked asgree are resclved and addressed.those
marked in yeltow areknown but solutions are work-ir-progress
Patient
Tssue
-Motion
predicted. The effect of flexible body coils on overall
PETattenuation was recently estimated by Tellmann and col-leagues
[64]. The authors report a maximum bias of 4 %
inattenuation-corrected torso PET if surface coils are notaccounted
for during AC. This bi as is negligible compared tothe respective
bias in head studies when ignoring dedicated,rigid head RF coils
(up to 20 %). MacOonald and colleaguesreport sirnilar results
[65].
All PETlMR vendors today offer CT-based attenuationtemplates for
rigid coils as well as for the patient bed that areseamlessly
integrared during the attenuation correction.Nonetheless, elinical
studies are required to further study theeffect of misaligred RF
coil ternplates and missing ternplatesfor aecurate represertation
of flexible coils on PETquantification.
The Presence of MR Contrast Agents
MR-based AC could potentially be biased from the presenceof MR
centrast materials. which are ypically made up ofiron oxide and
Gd-chelates for oral and intravenous (IV)applicaion, respectively,
It is known from the developmentof CT-based AC that the presenee of
contrast materials withatomic nurnbers higher than those of water
may lead tobiased attenuation maps for PET crnissior data. The
sameeffects may oecur with MRCA that are applied during PET/MR
irnaging, Furthermore, the presence of MR contrastagents may
produce changes in the vR signal intensity thatyield biased
attenuation maps. First studies indicate no nega-tive effect from
MR contrast on PET quantification follow-ing MR-based atteruatior
correction [66, 67J.
/' Transverse FOV
MR non-uniformities
-t
J *~i iiiiiii\: 7 / iiiiiii~~
Understccd and WIP
Limited FOV and Truncation Effects
Given the reduced bore diameter and the relatively
longexarninaion times in PETlMR cornpared to elinical PETICT, most
patierts are positioned in the more cornfortablepositior with their
arms dowr. Thus, the patient anatomymay well extend beyond the
transverse FOV of the MR(typically 50 cm). whereby the arms or the
trunk of thepatient are not fully eovered by the MR irnages used
forMR-AC. This may yield an underestimation of the recon-structed,
attenuation-corrected ernissior activity eoncentra-tion. Truncation
artifacts were deseribed for PET/CTimaging [68J and have been
reviewed for PET/MR [69]. ltwas shown that with the arrns extending
beyand the FOV ofthe MR the PET activity following MR-AC was biased
byup to 14 % in the area of truneation. The underestirnatedactivity
eoncentration could be recovercd to within 2 % ofthe norniral
concentratior following sirnple, manual exten-sion of the
atteruatior map.
An alternative solution would be to use the uncorreetedPET image
to estirnate the patient cross-seetion in thoseareas outside the
measured FOV where no - or only geo-metrically distorted - MR
information is available [70, 71].The elinical feasibility of this
approach still needs to be vali-dated. Particularly, in irnagirg
scenarios with highly specifictracers the arms may be difficult to
segment auornatically inthe uneorrected PET irnages.
Figure 2. 13 surnmarizes the challenges and the status
ofMR-based attenuation correctiors in PET/MR. Most chal-lenges are
understood with so me being addressed sufficientlyand some awaiting
further optimization, val idation and elini-cal adeption.
-
24T. Beyer et aL.
MR-Based Partial Volume Correction corrected
As early as 1991, Leahy et aL.suggested that PET reconstruc-tion
could be improved by using anatomical MR irnages fromthe same
patient as prior information [721 lt is common elini-cal practice
today for neurology patients with a PET-indicationto alsa undergo
an MR examination. However, MR-guidedPET reconstruction has not yet
made the transition fromre search into elinical routine. Aside from
logistical problemsassociated with the retrieval of the
complementary image sets,sub-optimal rerospective image aligrment
would significartlydeteriorate the qualiy of the PET data [731.
However, in com-bined PETlMR imaging systems, the sparial (and
ternporal)alignment accuracy could be improved, thus, helping to
pro-mote the concept of MR-guided PET image recorstruction.
Even if the PET image is reconsructed independertly ofthe MR
image, it is stili possible to use the MR image of thepatient as an
aid for improved quantifieation. In particnlarMR-guided partial
volume correction (PVC) was suggestedas early as in 1990 [74, 75].
Again PET and MR images fromcombined PETlMR examinations may
facilitate irnprove-ments in MR-based PET quantification through
the use ofMR-based PVc.
Uncorrected(18F)-FDG PET
.,.alP'
MR-Based Motion Correction
Fig.2.14 FDG-PET irragesfollowing auenuation correction (left)
andmotion + atenuation correction (riglt) clearly demonsrae the
poten-tially improved quality of the data from lengthy examinations
(Courtesyof J Scheins and H Herzog, Research Centre Jlich)
Patient motior. from involnt::ry movements, and cardiac aswell
as respiratory cycles, is a major contributior to degradedPET image
quality. In addition. patient motior will lead tolocal or extended
mis-alignment of complementary anato-metabolic image information.
In PET/CT, for example, thePET image is acquired over several
minutes, while the CTsean is amalter of seconds and frequenly
acquired during asingle breath hold. As a result, patiert moion
typicallycauses local rnisaligrment between the PET and CT
imagesand may lead to seriolis artifacts for AC, for example
nearthe diaphragrn. Dedicated breahing instructions have beenshown
to help reduce misalignment in the horax and upperabdemen [76. 77].
Other authors have reeommended 4DPET/CT acquisition and AC,
however, this involves a sub-stantially higher patient radiation
dose [78-80J.
Respiration is expected to generate misalignment andblurring in
PETlMR irnages, too. As MR seans generallytake nch longer than CT
seans, patients spend an even lon-ger time in the PETlMR compared
to PET/CT, and can se-quently patient motion is likely to cause eve
n more severeartifacts. However, integrated PET/MR system
technologyoffers a promising solution LO the problem (Fig.
2.14).Various NIRl motior-tracking techniqes are available
inelinical settings, including but not limited to cloverleaf
navi-gators [81J. Such echniques have been tested with theBrainPET
system with promising results from estimating
and correctirg involuntary head motior as a result of
relax-atior of neck muscles. Usiug 3D Hoffman brain phantomand
human volunteer studies. Catana et ol. reported thathigh-
temporal-resolution MRI-deived ma tion estimatesacquired
simultaneously on the hybrid BroinPET system(Siemens Healthcare)
can be used to improve PET imagequality, thus increasing its
reliability, reproducibility. andquantitative accuracy [SOL.
Likewisc. novel 30 cine sequences are under develop-ment to
track spatio-temporal deformation of organs such asthe heart and
the thorax, Subsequently, deforrnation fieldsare generacd and
incorporated ino the PET reconstnction
[51.82-85].Thus, the use of periodic MR navigator signaIs in co
n-
junction with a 4D model of the human torso may help tocorrect
for motion-induced image degeneration in PETlMRdata following
4D-MR-AC. which would be o major advar-age over CT-AC.
PET/MR Safety
Combined PET/CT has been clinically very successful andmay well
serve as a benchmark for the development of PET/MR. However,
despite the success of PET/CT the re are alsa
2 PET/MR lnstrumentation 25
shortcomings in the use of CT as the anatornical corrple-mert to
PET. As such, CT uses a souree of iorizing radia-tion for imaging
and. therefore, adds significart radiatiordose to the overall
exarnination. Brix et al, have shown thatthe diagnostic CT
contributes up to 75 % of the effectivedose in patiens undergoing
whole-body FDG-PET/CTexarninations for oneology indications leading
to a total ofabout 25 mSv effective dose [86]. These dose levels
mayraise concem in selected population like adolescens andfemales.
Figure 2. 15a illusrates the relative cortribution topatient
exposure from the individual steps in a combinedPET/CT
examinatioi.
In PETlMR examinations, overall patient exposure isreduced
significantly by replacing the CT imaging step withan MR imaging
sequence (Fig, 2.1Sb). In addition, MR pro-vides advaneed
functioral imaging information, such asDWI or MRS, wihout adding to
the overall radiatior expo-sure burden. Nonetheless, staff exposure
is expected toinerense slightly in PETlMR, given the complexity of
thepatient set-up when employing a range of surfaee RF bodycoils.
However, no valid data are available as of yel.
Long-tem experience and hundreds of millions of rou-tinely and
safely performed MR exarninations confinn thatMRI is a safe imaging
modaliy. Nevertheless, a number ofsafety concems do apply to PETlMR
as discussed by Brixet al. [871, of which all are all associated
with the generalsafety issues known from MR-only imaging. The
strong staticmagnetic field associated with MR systems potentially
canattract ferrornagneic equipment as well as same
patientirnplants. and accelerate these towards the strongest
magneticfield in the isocertre of the PETlMR system. In same
patientsthe strong and fast switching gradient fields may lead
toperipheral nerve stimulation that are harrnless but
neverthelessdisturbing. Finally, the strong-pulsed RF fields for MR
signalexeiration can cause tissue henring. As with all other RF
trans-mitting devices, the RF power n MR imaging is limited
loharmless values of the specific absorption rate (SAR) not
lead-ing to critical tissue heating. Some electric condeting
metalimplants, however, potentially may increase the local
SARvalues during an MR examination above the allowed SAR!imits. To
reduce all associated poential risks of MR imaging,patient
questionnaires and patient screening and selection pro-cedures have
to be established and used in daily routine.
Accordingly, MR and PETlMR examinations of patienswith passive
implans (e.g., vascular elips and clarnps, intra-vascular stents
and filters, vascular access ports and catheters,heart valve
prestheses. orthopedic prostheses, sheets andscrews, intrauterine
contraceptive devices), active irnplants(e.g., cardiae pace-makers
and defibrillators, cochlearirnplants, electronic drug infusion
pumps) or other objectsof ferromagnetic or unknown material
(pellets, bullets) arealways associated with a potential risk.
Careful pre-exarni-nation interviews of the patients regarding the
presence or
absence of passive implants, which may interfere with theMR
imaging protocol, or deter the patient from this examina-tion all
together is mandaory [87J.
Summary and Conclusion
Multi-modality imaging insrumentation has evolved dra-rnatically
during the past decade. Combined SPECT/CT,PET/CT and, lately,
PETlMR have revolutionized imagingand medical diagnosis. In these
times of limited resources inhealthcare and rapidly increasing
radiotion awareness, anypredietions for fuure developments of
PETlMR technologymust take into account a variety of aspects,
ranging fromcost-effectiveness to overall radiation dose. While
techno-logical innovation, such as PETlMR. always pairs
withenthusiasm and public interest. subsequen commercial sys-tems
must be affordable and srategies for the ir elinicalimplementation
must be assessed for their health benefit tojustify their pursuit
within alocal or global healthcare sys-tem [88]. The impressive
advances in imaging technology ofthe past decade came at a cost.
but at what point do theseadvarces become cost-effective?
Whole-body PET exami-nations that took i h at the start of the last
decade now take5 min on PET/CT; the actual imaging takes only o
fraction ofthe time needed for patient preparation and positioning
orreporting the study. Does the increased wealth of
availableinfannation from the MR rnake up for the increased
exami-nation time?
The radiation dose to the patiert incurred by PET/CT iselearlyan
issue. Although the ALARA (as low os reasonablyachievable)
principle is sond advice, there are elearly groupsof
cancer-sufferers such as those in children and young adultswhere
the probability of inducing a second. radiaion-associared cancer
exceeds the benefits that can be accruedfrom the study. Different
imaging stratcgies should then beadopted, such as MR!, optical
imagirg or ultrasound.
The cornrnendable drive to reduce radiation exposure topatierts
has fostered an interest in a combination of PETwith MR!. However.
it is fair to assume that as long as dis-eases such as cancer and
dementia rernain primarily diseasesof the elderly, the benefis of
nuclear and X-ray imaging willlargely outweigh the risks.
Will the coming decade witness the replacement of PET/CT by
PETIMRI? Same believe it wilL.just as in the 1980sthe re wcre those
who predicted that MRI wold replace CTwithin 5 years. Of course
that never happered, as both tech-niques have strengths and
weaknesses and they have eachfound their niche in the medical
imaging armamentarium.The same is likely true of PET/CT and
PETIMRI-thetechnical challenges will be solved and simultaneos
acqui-sition of MRI and PET will undoubtedly open new doors
inelinical research and eventually alsa in the clinic.
,1J
-
26
a ..
1.
1. Patient prepartionJpositioning
2. Topogram
3. Low-dose CT
~ cr- ~OO attenuation ccrrection4. Multi-step PET
5. Dedicated CT (contrast, 9ating, breath- hold)
6. Escorting the patient out
b
1. Patient preparationlpositioning
2. Scout (MRI)
3. + 4. Multi-step PET/MR include. MR-AC (3)
5. PET and MR image reconstruction,Escorting the patient out
Fig.2.15 Relative conrnbutions to patient and staff exposure
during nwhole-body oneology examination in PET/cr Ca) and
PET&.>IR (b).The amount of radioactivity injected into the
patient for a PET/cr (a.step i) and PETI?v1R (n. step i) is assuned
to be identica!. Note. patient
Acknowlcdgcmcnt Wc are indebted to Caspar Delso (GEHe),
HansHerzog (Research Centre Jlich). Jens-Christoph Georgi
(SiemensHealthcare). Antenis Kalemis (Philips Healthcare). Bernd
Pichler(University of Tbingen). Nina Schwenzer (University of
Tbingen).
T. Beyer et a'
-ii
i- irel.exposure patient Staff
--i
rel. exposure Patient Staff
set-up in PETlMR is more elaborate and. therefore, relative
andpotentially total sraff cxposures are expected to be higher than
those inPET/CT
Jrgen Scbeins (Research Centre Jlich). Hclger Schnidt
(Universityof Tbingen). David W Townsend (Singapore). Rtiner Veigel
(PhilipsHealthcare). Patrick Veit-Haibach (Zurich) for helpful
discussions andthe provision of support materials.
2 PET/MR Instrumentation
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Technical Principles and Protocolsof PET/MR Imaging 3A. Kalemis
~
Contents IntroductionIntroduction .. 29
30
30
31
31
PET and MRl are two well-established medical imagingmodalities
that are used frequently os diagnostic tools in awide range of
elinical irdications providing complementaryinfannation [Il. MRI
can provide anatomical informationwith very high spatial
resolution. and functional measurernentsat organ and tissue level
with high diagnosric sensitivity. Onthe other hand, PET images
functional processes at cellularand sub-cellular level with very
high diagnostic specificityand high tracer detection sensitivity (I
0-11-~ 0-12 mol/l) [2] butwith a sparial resolution inferior to
MR!. This complementarymarching of capabilities renders both
PET(lCT) and MR] nec-essary in sevemi disease pathways,
particularly in oneologyand neurology. A significant workflow
lirnitation, when bothmodalities are needed, is the physical and
organisationa! sepa-raion of the two systems [3]. Patients who need
both PET andMR imaging are of ten referred for such seans
independentlyand. often, theyare scanned with a significart time
difference.This renders the fusion of information from both
exarninationsdifficult or even impossible due to disease status
changes orseveralother technical and organisational factors 14].
Hence,when MRl is the preferred imaging rnodality versus CT, PET/MR
should be more clinically useful than PET/CT and a com-bined single
examination could provide significant benefits toboth the patient
and the hospital.
Today, there are two different designs for combined PET/MR
sysems: positioning PET inside the MR! magne or intandem, similar
to PET/CT [5J. Both philosophies attemptto balance elinical
utility, user flexibility in developingclinically-relevant
PETlMR-dedicated imaging protocols,potertial sacrifkes in relation
to stand-alone state-of-the-artPET and MRI imaging capabilities and
co st. lrrespectively,of the design and techrical differences with
the stand-al onesystems, PETlWIR, as a novel imaging option,
requiressignificant innovation at various levels n order to
surpasscurrent concems and scepticism. The latter are of elini-caL.
organisatioral and techrological nature. Questiorsabout how it
compares with PET/CT, under which elinical
Opera tion al Requirements ..
PET/MR Applicatiors ...
Clinical ProtoeoIs and Workflow Considerations ..
Single-Organ Imaging
Whole-Body lmaging .. 32
Tcchnical Requirements
Sean
34
34
35Image Quality and Quantifieation ..
37
A. KalemisPETIMR,.Philips Healthcare.Guildford Business Park.
Guildford, Surrey GU2 8XH. UKe-mail: [email protected]
._~__ O. Ranb et aL. (cds.). Ar/as o/PET/MR Imagig in
Oneology.....:::: 001 10.1007/978-3-642-31292-2_3, Springer- Verlag
Berlin Heidelberg 2013
29
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30A. Kalemis
seenarios either of them should be used, patient throughput.ease
of worktlow, building and running costs, ownership ofdevice and
staff (betweer different departrnents). reliabilityof new the
technologyand image quality in comparison tostand-alane systems are
com mo n discussion points.
Operational Requirements
l is common in healthcare for radcal new technologies torequire
a second innovation wave, in the area of its structureand
organisation. in order to optirnise their contribution indisease
managernent and patient pathway [6J. An organisa-tion wishing to
adopt PETlMR, in particular, has to over-corne its complicated
logistics, of ten the single-modalitytrained technical personnel,
the saffing requirement fromtwo different departments and the
excessive scanning timerequired to acquire both PET and MR seans,
which raisessignificantly the cost of each sean. At this level,
institutionsare requested to innovate in order to successfully
adopt thisnovel technology.
At infrastructure level, so me degree of
cross-departmentalcollaboration and in sorne cases even
restructuring is neces-sary to bring the Nuclear Medicine (NM) and
Radiologystaff much c1oser. Dual-raining of the technieal personnel
isessential in order to operate the scanner while arother
long-terrn consideration is the need to eress-trnin Radiologists
andNuclear Medicine physicians from both modalities [7J.However,
such needs are not sttuightforward due several fac-ters. amongst
ohers the exising territorial and protectivepractices in many
healthcare faciluies and the variatiors inthe legal frameworks for
imaging technologists [8L In manyhospitals Radiology and NM
departments are far from eachother creating further complications
in staff allocation and/orlogisucs of the new scanners, Therefore,
a suecessful modelneeds to be devised for the placement of the
scanner corsid-er